1. Field of the Invention
The present invention relates to a carrier for use as a two-component developer in image-forming apparatuses such as a digital copying machine and a printer and, more particularly, to a binder carrier containing magnetic particles dispersed in a binder resin.
2. Description of the Related Art
In electrophotographic apparatuses, such as copying machines and printers, the process of two-component development utilizing a two-component developer comprised of a toner and a magnetic carrier has been in general practice for development of an electrostatic latent image formed on an image-supporting member, such as a photosensitive member.
A variety of carriers for two-component developer are known including iron powder carriers, ferrite carriers, resin-coated carriers containing those magnetic particles coated with resin, and binder carriers having magnetic fine particles dispersed in binder resin. However, resin-coated carriers involve a problem such that when the carrier is transported in the form of a magnetic brush on a developing roller, the crest of the brush is so hard as to prevent the formation of a smooth image. Another problem is that scraping operation of the magnetic brush may cause the trouble of toner image disturbance and may damage the photoreceptor member. Resin-coated carriers involve a further problem such that as the carrier is used as such, its coat layer tends to peel off, so that the useful life of the developer is rendered short.
Binder carriers are receiving considerable attention because such a carrier can not only solve above noted problems, but is readily adaptable for particle size reduction and has high electric volume resistivity so that it is hardly injected by electrical charges from the developer-supporting member.
A conventional binder carrier, however, has such a problem that in the course of use of the binder carrier, toner components, such as a charge-controlling agent and an after-treatment agent, tend to adhere to the surface of the carrier, thus causing deterioration of the charging capability of the carrier, with the result that the toner cannot be sufficiently charged. This leads to the occurrence of image noise, such as fogging, with respect to the image formed on a copy sheet.